22.1 - Alpha, Beta & Gamma Radiation

Radioactive decay and the types of nuclear radiation

• Some isotopes have unstable nuclei. An unstable nucleus tends to break down, or decay, releasing radiation as it changes into a more stable arrangement.
• This process is called radioactive decay, and a substance whose nuclei decay this way is radioactive.
• A nucleus is unstable when there's an imbalance between the numbers of protons and neutrons it contains (for example, too many neutrons), or when it is too heavy.
• The radiation comes from the nucleus itself.

The three types of radiation

1. Alpha particle (α) - Two protons and two neutrons, the same as a helium nucleus. Relative mass 4, relative charge +2. Comparatively large, heavy and slow-moving.
2. Beta particle (β) - A fast-moving electron created in, and thrown out of, the nucleus when a neutron turns into a proton. Virtually no mass and a relative charge of -1. Small and travels quickly.
3. Gamma ray (γ) - A high-frequency electromagnetic wave released by the nucleus. Carries away energy but has no mass and no charge.

Effect on the element

• Emitting an alpha or beta particle usually changes the nucleus into a different element.
• Gamma radiation carries only energy, so it leaves the element unchanged.

Ionising radiation and ionising power

• Ionising radiation knocks electrons off atoms it passes, turning those atoms into positively charged ions.
• Ionising power measures how readily a source does this.

What affects ionising power

How strongly a type of radiation ionises depends on its charge and its energy:

• Charge - A more highly charged particle attracts or repels an atom's electrons more strongly, so it's more likely to remove electrons.
• Energy - A particle carrying more energy can transfer more energy to an atom it collides with, making ionisation more likely.

Ionising power of each type

• Alpha - Strongly ionising. It carries much more mass and energy than beta and has the largest charge (+2), so it collides with and ionises many atoms over a short distance.
• Beta - Moderately ionising.
• Gamma - Weakly ionising. With no mass and no charge, it tends to pass straight through atoms rather than collide with them.

Penetration and range of radiation

The more strongly a type of radiation ionises, the more it collides with atoms, so the shorter its range and the more easily it's absorbed.

Range and what stops each type

• Alpha - Travels only a few centimetres in air and is stopped by a sheet of paper.
• Beta - Travels up to a few metres in air and is stopped by a thin sheet of aluminium (around 5 mm thick).
• Gamma - Travels a long distance through air and is only absorbed by thick lead or metres of concrete. Gamma is never completely stopped; thick lead and concrete reduce its intensity rather than blocking it entirely.

Deflection of radiation in electric and magnetic fields

• Alpha and beta particles carry electric charge, so they feel a force in an electric or magnetic field and are deflected.
• Alpha is positive and beta is negative, so the two are deflected in opposite directions.
• Gamma rays carry no charge, so they're not deflected by either field.

In an electric field

• Between two oppositely charged plates, a beta particle is deflected towards the positive plate and an alpha particle towards the negative plate.
• Alpha is deflected less than beta because its much greater mass makes it harder to deflect.

In a magnetic field

• Alpha and beta are deflected by a magnetic field, provided they aren't travelling parallel to the field lines.
• The direction of deflection can be found using Fleming's left-hand rule, where the motion of a positive alpha particle represents the direction of conventional current.

Alpha and beta decay change the element

• Emitting an alpha or beta particle changes the number of protons in the nucleus, turning it into the nucleus of a different element.
• Gamma emission doesn't change the number of protons, so the element stays the same.